Fiber optic cables include at least one optical fiber that can transmit data, computer, and telecommunication information. Self-supporting fiber optic cables are designed for aerial applications and typically include a messenger wire and a core section having conductors therein which may be solely optical, or a combination of optical and electrical conductors. Self-supporting fiber optic cables of the "figure 8" type may be characterized into two general categories, namely, self-supporting cables with a core section having no excess length relative to the messenger wire, and self-supporting cables having a core section having an over-length, typically about 0.2%, relative to the messenger wire. Examples of known self-supporting cables having no core section over-length are disclosed in U.S. Pat. No. 4,449,012, U.S. Pat. No. 4,763,983, U.S. Pat. No. 5,095,176, and U.S. Pat. No. 5,371,823. Examples of known self-supporting cables having a core section over-length are disclosed in U.S. Pat. No. 4,662,712 and U.S. Pat. No. 4,883,671.
When installed in a self-supporting application, self-supporting cables may experience a high degree of tension. The messenger wire bears most of the tension, thereby supporting the core section, and protecting the optical fibers in the core section from high tensile forces. As tension acts on the messenger wire, however, the messenger wire tends to elongate which results in an elongation of the core section. Elongation of the core section of a self-supporting fiber optic cable not having an over-length may cause attenuation losses in the optical fibers in the core section. On the other hand, where the core section of a self-supporting cable having a core section over-length is elongated, the elongation is, up to the amount of existing over-length of the core section, advantageously taken up by the over-length in the core section whereby the core section in may be elongated without potentially causing strain and/or attenuation in the optical fibers.
Several methods of manufacturing self-supporting fiber optic cables having a core section over-length have been developed, for example, by sag formation, thermal/tensioning, and speed differential methods. In the sag formation method, for example, as is disclosed in JP-8-136778 and JP-8-211260, the core section is given an excess length in the form of sagged portions and then the messenger and the core section are bound together at spaced intervals by a wire. As an alternative to binding with wire, plastic clamps may be over-molded about the messenger and the core section, for example, as is disclosed in JP-61-29811, U.S. Pat. No. 4,883,671, and U.S. Pat. No. 4,662,712.
The thermal/tensioning or speed differential methods can be used to create the over-length. An example of the thermal/tensioning method for forming the over-length is disclosed in JP-9-54232, wherein the core section over-length is created by a heater disposed between a capstan and a brake. The heater heats the messenger wire and causes it to thermally elongate while the capstan and the brake simultaneously apply tension to the messenger wire to mechanically elongate the wire. The elongation created in the messenger wire is therefore the sum of the incremental elongations made by the thermal and mechanical elongations of the messenger wire. Alternatively, the speed differential method as disclosed in JP 8-75969 can be used to create the over-length. This method requires the core section to have a slightly faster speed as compared to the messenger section as the jacket material is extruded thereover. The difference in speed creates the over-length in the core section.
When practicing the thermal/tensioning or speed differential methods of over-length formation, a cable jacket may be simultaneously extruded about the messenger wire and the core section with intermittently formed webs connecting the messenger wire and core sections. Prior art FIGS. 1-3 show an exemplary self-supporting cable 10 that can be made by, for example, the thermal/tensioning or speed differential methods. Self-supporting cable 10 includes a messenger section 12 having steel wires 13, and a cable core section 14 having a slotted core 15, optical fiber ribbons 16, and a water absorbent tape 19 wrapped therearound. Cable core sections 12,14 include an extruded jacket 17 having a web 18 that connects the messenger and cable core sections together. Cable core section 14 can have an over-length of about 0.2% relative to messenger section 12 whereby cable core section 14 can have at least one undulation between webs 18 (FIG. 3).
The extruder used to manufacture self-supporting cables with intermittently formed webs may include a plunger, e.g., as is disclosed in JP-46-38748 and JP-8-75969. As disclosed in JP-8-75969, for example, the extruder head includes a melt cavity with a molten jacketing material therein. With reference to the manufacture of cable 10, as the messenger wires and core translate through the melt cavity they are coated with the molten jacketing material. As the messenger wires and core exit the extruder head, a die orifice determines the peripheral shape of the cable jacket therearound, and the orifice includes a web-forming area for the formation of webs 18.
The plunger operates by moving into a blocking position in the die orifice between cable sections 12 and 14, and physically blocking the molten jacketing material from forming web 18. FIG. 1 is a cross section of cable 10 where the plunger blocked the jacketing material and a corresponding longitudinal gap exists. The plunger is reciprocated in and out of the blocking position so that webs 48 are formed intermittently, spaced by longitudinal gaps. FIG. 2 depicts a cross section of cable 10 as made when the plunger was in a retracted, non-blocking position whereby web 18 was formed.
The present inventors have discovered a problem with the use of plungers that is not addressed in the foregoing background art. Namely, as the plunger is reciprocated between the blocking and non-blocking positions, the pressure of the molten jacketing material in the melt cavity can fluctuate about 50 p.s.i. or more. The pressure fluctuation in the melt cavity causes an uneven application of the jacketing material about the core section, resulting in the formation intrusion zones 17a (FIG. 1). The intrusion zones are formed in the lengths of cable corresponding to the existence of the longitudinal gaps, and the intrusion zones virtually disappear when the plunger is in the non-blocking (web-forming) position (FIG. 2). Intrusion zones 17a can be characterized by the disadvantageous reaction of the molten jacketing material to the pressure fluctuations, i.e., the jacketing material is pushed toward the slots of core 15, and possibly forces the waterblocking tape against the ribbon stacks. When this occurs, forces acting on a ribbon stack can warp and/or disintegrate the stack structure as shown by disheveled stacks 16a (FIG. 1), and can cause undesirable microbending or macrobending of the optical fibers in the ribbons. The presence of intrusion zones 17a can negatively affect the optical performance of the cables in the final product. Moreover, after the cable is installed in the field, optical performance losses can be increased by temperature cycling and mechanically induced stresses by forcing the intrusion zones deeper into the slots. The intrusion zones can potentially counter any built-in stress avoiding benefit of the core section over-length.